docs: def _get_b(self):
          """Return my Y-intercept.
  
          I probably don't really hit the left edge of the window, but
          this is where I would, if I were long enough.
  
          """
          orig = self.origin
          dest = self.destination
          (ox, oy) = orig.pos
          (dx, dy) = dest.pos
          denominator = dx - ox
          x_numerator = (dy - oy) * ox
          y_numerator = denominator * oy
          return ((y_numerator - x_numerator), denominator)